You are making sweeping assumptions:
Each file has been written exactly once
That's not true.
nothing has ever been deleted or rewritten
Also not true.
only 1 process at a time ever creating new files
Definitely not true.
Windows is a complex OS with multiple processes and threads running simultaneously, many of which write to the hard drive simultaneously especially during OS setup, installation of updates, and installation of applications.
A lot of fragmentation occurs with log files, as they are naturally appended to. Modern Windows does heaps of logging, far more than you may think, so that will account a fair amount of the fragmentation you're seeing.
Also when it comes to updates, Microsoft employs sophisticated delta update technology that patches parts of files which can easily result in fragmentation.
Then there's the .NET Optimization Service which pre-compiles .NET assemblies in the background (especially during install/update of .NET). This background activity causes fragmentation as it goes on while other activities are taking place.
I could go on and on, but I'm sure you get the idea.
The fragmentation you see is totally normal in modern Windows, which is why it's good practice to defragment after an OS install on non-SSD hard drives.